1. Field of the Invention
The present invention relates to a cooling device of a computer rack and a computer installation comprising such a device.
2. General Background
A computer rack is a hardware structure in which computer equipment is arranged (computers, storage devices, etc.), generally identified by their vertical position in the rack. The standard U unit has thus been defined by the standard EIA-310-D to define this identification in a universal manner: 1 U=44.45 mm.
The computer equipment of a rack comprises electric power components whose operation generates a dissipation of heat that must be evacuated. Although the emergence of CMOS technologies has enabled the power dissipated by components to be reduced and their capacity of integration on a single chip to be increased, the applications require in parallel ever greater computation power (database management, decision-support applications, supercomputing, Internet access, etc.).
Hence, the techniques for designing computers with Symmetric MultiProcessing architecture (SMP architecture) and the techniques for grouping computers into server clusters enable a response to this need for power to be made. It is thus possible to design HPC type computers (High Performance Computing), capable of integrating up to ten thousand basic processors at very high clock frequencies, distributed in a large number of computation servers arranged in several racks, themselves interconnected with each other in a closed and air-conditioned computer room in which attempts are still made to incorporate a maximum of servers in the three dimensions. The aim is to reach a maximum of Giga Flops (Floating-point Operations Per Second) per m2 and/or the maximum of Internet connections per m2.
However, at the same time, the use of increasingly refined technologies to produce transistors and the race for high frequencies has led to the increase of leakage currents of the transistors and to an explosion of power dissipated by each processor, which can now reach 185 Watts. Hence, to fill a computer room necessarily stumbles at the first limit reached among the following three: its capacity in volume, its capacity in electric power and its capacity to evacuate the heat generated.
The evacuation of heat generated by an air-conditioning of the computer room poses significant problems of costs: costs related to the electric consumption of the air-conditioning system, but also those related to its installation and maintenance.
Moreover, a computer room generally comprises a raised floor constituted by square tiles of dimensions 60 cm×60 cm, on which the computer racks are arranged and under which electrical connection cables travel. A standard computer rack has a height generally comprising 42 useful U, a width equal to that of one tile and a variable depth, but generally comprised between one and two tiles. Moreover, from a maintenance and wiring point of view, it is necessary to have access to the raised floor under the computer rack, such that the tile in front of the front panel of the rack and the one in front of its back panel must be free to access. Hence, the surface area of a computer rack necessary for a computer room is in fact four tiles, which represents the standard reference. This is why the depth of a computer rack must not exceed two tiles, any overlapping on a “maintenance” tile condemning it.
Considering the above, the maximum density ratio possible with standard hardware in a computer room is therefore 42 U/1.44 m2 which gives a ratio of 29 U/m2, under the hypothesis that all the space within the computer racks of the room is used. From this density ratio, it is possible to estimate the thermal dissipation ratio per room surface area, that thermally governs the cooling capacity of a room by taking into account operating criteria of the computer hardware (i.e. limit of air inlet temperature in the hardware).
One cooling solution is based on the fact that the computer equipment arranged in the rack is self-ventilated and thus cooled, owing to the air-conditioning of the computer room. The cooling circuit is therefore as follows: the cold air supplied by the air-conditioning of the room enters the rack via a front door having a certain percentage of opening, heats up by passing through the computer equipment and is expelled from the rack via a rear door in the room with a higher output temperature, sometimes by several tens of degrees according to the devices. As each item of computer equipment can only operate in a range of air inlet temperatures, the air-conditioning of the room must regulate the ambient temperature in a sufficient manner to remain within this range.
Given that an efficient air-conditioning can cool computer racks that dissipate up to 10 or 12 kW, this gives a thermal dissipation ratio of 7 to 8 kW/m2. If account is taken of the actual thermal load of a server that is comprised between 750 W and 1 kW per U, a thermal dissipation ratio comprised between 7 and 10 U/m2 is obtained and therefore the room, which has a theoretical capacity of 29 U/m2, can cool the computer racks provided that they are filled at a rate comprised between 24 and 34%.
In other words, for a computer room to be cooled correctly by air, it must only be constituted of racks that are ⅔ or ¾ empty. Consequently, the economic yield is low in terms of surface area, in terms of wiring (length, distance), as well as in terms of power consumption of the air-conditioning system, which will further have to cover the lack of uniformity of the ambient temperature of the room if the equipment is mixed.
Another cooling solution consists in reducing the thermal problem in the space of the rack, that is to provide cooling means in the computer rack itself. This cooling means comprises for example an air/water heat exchanger. The cooling circuit is therefore as follows: the cold air supplied by the air-conditioning of the room enters the rack, heats up by passing through the computer equipment and is cooled by the air/water heat exchanger installed in the rack, the latter being capable of transferring the heat of the air to water exchanger that is injected back into a hydraulic circuit of the room and is cooled at the level of cold units. This solution is advantageous because the heat conducting property of water is around 4,000 times greater than that of air.
However, the cooling means specific to each computer rack takes up space and the choice of its location is important.
For example, a first variant consists in arranging this cooling means laterally in the rack. The disadvantage of this variant is that to integrate it in the side of the rack, the dimension of the rack must be increased in width. It then no longer corresponds to a tile in width, but to at least one tile and an additional ⅓ of a tile. If the U/m2 ratio is recalculated, a reduction of 22 U/m2 is observed, which gives a thermal dissipation ratio of 16.5 to 22 kW/m2, less than the ratios calculated in optimum conditions.
A second variant consists in arranging this cooling means at the bottom in the rack. The disadvantage of this variant is that to integrate it in this manner, a certain height must be reserved for it and therefore reduce the vertical integration capacity of computer equipment of the rack. In the same manner as previously, if the U/m2 ratio is recalculated, a reduction of 23.6 U/m2 is observed, which gives a thermal dissipation ratio of 17.5 to 23.6 kW/m2, still less than the ratios calculated in optimum conditions.
In fact, it appears that the best solution is to arrange this cooling means at the back of the computer rack, at the level of its rear door, in a space available owing to the fact that the computer racks have a depth less than two tiles.
The invention thus applies more particularly to a cooling device of a computer rack equipped with a back panel comprising an evacuation zone, toward the exterior of the rack, of air having circulated over electric power components arranged within this computer rack, comprising a rear door in the thickness of which air cooling means is arranged.
Such a device is described in the document published under the number US 2006/0232945. In this document, the cooling means of the computer rack comprises an air/water heat exchanger integrated in the thickness of the rear door of a computer rack. The rear door is mounted on the chassis of the computer rack. But it imposes a specific chassis different from a standard rack chassis not comprising specific cooling means. Moreover, if the rear door was also to comprise a ventilation system, it would be even heavier and/or more voluminous and a different chassis structure would also still have to be designed.
It thus appears that a rack initially designed to receive slightly dissipative hardware would have to be replaced by another rack such as the one described in the aforementioned document if the hardware it contains evolves and becomes more dissipative. Now, a computer room is led to evolve over time: to be obliged, either to modify the air-conditioning system, or to replace the computer racks according to the variation of needs, is restrictive. It is further important to be able to conserve a variety of equipment in a room without for as much harming the thermal uniformity of the room, which is not simple to provide with the aforementioned racks.
It can be advisable to provide for a computer rack cooling device that can overcome at least one part of the aforementioned problems and constraints.